The present invention relates to a method of forming an element isolation region in a semiconductor device.
In a silicon semiconductor integrated circuit, an active area prospectively serving as an element is surrounded by an element isolation region covered with a relatively thick field oxide film to be isolated from other active areas.
As a method of forming this field oxide film, a poly-buffered LOCOS (Local Oxidation of Silicon) method is available.
FIGS. 6A to 6K explain this poly-buffered LOCOS method.
First of all, as shown in FIG. 6A, a thin oxide film 21 is formed on the surface of a silicon substrate 1. As shown in FIG. 6B, a silicon layer 31b consisting of undoped polysilicon is formed on the thin oxide film 21.
As shown in FIG. 6C, a silicon nitride layer 4 is formed on the silicon layer 31b.
As shown in FIGS. 6D and 6E, a resist pattern 5 is formed by using a lithographic technique. The silicon nitride layer 4 is partly etched by using this resist pattern as an etching mask to form a silicon nitride mask 4a.
Subsequently, the resist pattern 5 is removed, as shown in FIG. 6F, and the silicon layer 31b as a buffer layer and the silicon substrate 1 are selectively oxidized by thermal oxidation using the silicon nitride mask 4a to form a thick oxide film 22c, as shown in FIG. 6G.
In this case, owing to the presence of the silicon layer 31b, the stress acting on the silicon substrate 1 is reduced. In addition, the stress produced in the silicon substrate 1 is also reduced by decreasing the oxidation amount of the silicon substrate 1 in forming a field oxide film.
By removing the silicon nitride mask 4a and the remaining silicon layer 31b laid without being oxidized under the silicon nitride mask 4a, an element isolation region covered with the thick oxide film 22c is formed, as shown in FIG. 6J.
In the poly-buffered LOCOS method described above, the silicon layer 31b in the region to be selectively oxidized is used without being etched. However, this method is not limited to such a technique, and the following technique may be used.
FIGS. 7A to 7L explain steps in forming a field oxide film by another example of the poly-buffered LOCOS method.
First of all, as shown in FIG. 7A, a thin oxide film 21 is formed on a silicon substrate 1. As shown in FIG. 7B, a silicon layer 31c consisting of undoped polysilicon is then formed on the thin oxide film 21.
By inserting the undoped polysilicon film between the thin oxide film 21 and a silicon nitride mask 4a (to be described later), the stress acting on the silicon substrate 1 in performing selective oxidation can be reduced.
As shown in FIG. 7C, a silicon nitride layer 4 is formed on the silicon layer 31c.
Subsequently, as shown in FIGS. 7D to 7F, a resist pattern 5 is formed by using a lithographic technique. The silicon nitride layer 4 and the silicon layer 31c other than the portions laid below the resist pattern 5 are sequentially removed by an etching technique using the resist pattern 5 as a mask.
And as shown in FIG. 7G, the resist pattern 5 is removed. After removing the resist pattern 5, as shown in FIG. 7H, a thick oxide film 22c is selectively formed on the silicon substrate 1 by thermal oxidation using the silicon nitride mask 4a as a mask.
Owing to the presence of the silicon layer 31c, the stress acting on the silicon substrate 1 is reduced.
When the silicon nitride mask 4a and the remaining silicon layer 31c laid without being oxidized under the silicon nitride mask 4a are removed, an element isolation region covered with thick oxide film 22c is formed, as shown in FIG. 7K.
The following problems are posed in the method shown in FIGS. 6A to 6I.
In the poly-buffered LOCOS method described above, as shown in FIGS. 6F and 6G, the silicon layer 31b consisting of undoped polysilicon is inserted between the thick silicon nitride mask 4a and the oxide film 21. For this reason, when the silicon layer 31b and the silicon substrate 1 are selectively oxidized by using the silicon nitride mask 4a, oxide regions called bird's beaks are formed at two positions between the silicon substrate 1 and the silicon layer 31b and between the silicon layer 31b and the silicon nitride mask 4a, as shown in FIG. 6G.
As a result, the cross-section of the field oxide film at the boundary of the element isolation region exhibits an overhang structure immediately after selective oxidation. In the subsequent steps, e.g., a gate electrode formation step, inconveniences such as disconnection at a stepped portion and non-etched residual portions occur.
Furthermore, in this poly-buffered LOCOS method, after selective oxidation of the silicon substrate 1, a void (hole) 9 may be formed in a portion, of the silicon layer 31b, on which stress intensively acts, as shown in FIGS. 6G to 6I. If this void 9 is formed, the thin oxide film 21 exposed to the bottom of the void 9 is etched when the silicon layer 31b is removed after selective oxidation. As shown in FIGS. 6J and 6K, when this thin oxide film 21 is etched, the silicon substrate 1 exposed through the void 9 may be etched. In such a state, if a diffusion layer is formed in a region including the etched portion of the thin oxide film 21 in the subsequent steps, the etched region may cause junction leakage. In addition, if a MOS gate electrode is formed on the thin oxide film 21, a normal channel cannot be formed, and a gate oxide film defect may be caused as well.
In order to prevent the formation of the void 9, it is conceivable that the thin oxide film 21 may be thickened. If, however, the thin oxide film 21 is thickened, a bird's beak region, which should be reduced, expands to reduce the effect of the poly-buffered LOCOS method.
In addition to the above problems, this poly-buffered LOCOS method involves a problem of the unevenness of the boundary (bird's beak end) of the field oxide film area between the active area and the thick oxide film 22c.
In this poly-buffered LOCOS method, in selectively oxidizing the exposed silicon layer 31b, since the oxidation rate is dependent on the plane orientation of each crystal grain of the silicon layer 31b, lateral oxidation from an end of the silicon nitride mask does not progress uniformly.
For this reason, as shown in FIGS. 6I and 6K, the boundary of the field oxide film area between the active area and the oxide film 22c becomes uneven. This makes it difficult to define a fine active area.
In addition, owing to the unevenness of this boundary, the breakdown voltage of a gate oxide film formed in the active area may vary.
Furthermore, in forming a gate electrode of a fine MOSFET with a size of 0.25 .mu.m or less, this uneven boundary adversely affects pattern formation by lithography.
Similar to the above example, another example of the poly-buffered LOCOS method involves the following problems.
One of the problems is that a void (hole) 9 is formed in a portion, of the silicon layer 31c, on which stress intensively acts after selective oxidation of the silicon substrate 1, as shown in 7H.
If the void 9 is formed, as shown in FIGS. 6J and 6K as well, the thin oxide film 21 exposed to the bottom of the void 9 is etched when the silicon layer 31c is removed after selective oxidation.
Subsequently, as shown in FIGS. 7K and 7L, the silicon substrate 1 itself is exposed and etched. As a result, a hole 9a is formed in the silicon substrate 1.
As described above, if the thin oxide film 21 as a pad oxide film is thickened to prevent such a hole 9a, the bird's beak region, which should be reduced, expands to reduce the effect of the poly-buffered LOCOS method.
In addition, as shown in FIGS. 6I and 6K as well, the boundary (bird's beak end) of the field oxide area between the active region and the oxide film 22c becomes uneven, as shown in FIGS. 7J and 7L.
As in the above example, this uneven boundary makes it difficult to define a fine active area.